Driving is a highly dynamic activity where a driver's awareness of the traffic environment may play an essential role in performance. Easy, smooth driving may depend on the driver's ability to develop situation-specific expectations. In some circumstances, however, infrequent or unexpected situations may not be taken into account. Understanding these unexpected situations may provide important insight on driver situational awareness and accident prevention. For example, braking is one of the fundamental driving maneuvers, as drivers use braking to control vehicle speed, maintain the distance to the frontal traffic participants, and stop at traffic lights, among other uses. Braking is also closely related to a driver's awareness of the surrounding traffic situations. In most instances, a driver's braking may be less (or moderately) intense indicating routine driving events. However, in other instances, a driver's braking may be more intense indicating a hard braking event, which may be the result of an intentional maneuver, e.g., decelerating to make a turn or exit from a highway, or may be the result of an unexpected situation, e.g., decelerating to avoid an expected object on the roadway.
Despite the popular dichotomy of human awareness control, driving often involves top-down and bottom-up processes, where the top-down processes are associated with prediction using learned knowledge, and the bottom-up processes relate to reaction to salient features in the environment. Furthermore, handling unexpected events conventionally involves a bottom-up process, while implementing planned actions conventionally involves a top-down process. In a naturalistic driving condition, the difference between the unexpected events and implemented planned actions may be captured using driver physiological signals. Current methodologies, however, do not correlate driver physiological signals to distinguish between different types hard braking events, and/or further analyze road features to determine what may have been the cause of the different types of hard braking events.